1. Field of the Invention
This invention relates to a two-step process for the production of polyamides from .alpha.,.omega.-dinitriles and .alpha.,.omega.-diamines. More specifically, the invention relates to a process for preparing polyamides wherein hydrolysis of the dinitrile is achieved by addition of water with simultaneous purging (i.e., venting) of water vapor prior to polymerization by addition of diamine to the hydrolysate.
2. Description of the Prior Art
Polyamides are conventionally prepared by the condensation polymerization of a diacid, such as adipic acid, and a diamine such as hexamethylene diamine, or by the polymerization of lactams such as .epsilon.-caprolactam. Other processes are known which involve preparation of polyamides by reaction from .alpha.,.omega.-dinitriles and .alpha.,.omega.-diamines in the presence of water. For example, Greenewalt U.S. Pat. No. 2,245,129 discloses a two-stage process in which a reaction mixture comprising a dinitrile, diamine, and water is heated in a first stage in a closed reaction vessel until a low molecular weight polyamide is formed, followed by heating at atmospheric or reduced pressure to remove liberated ammonia and excess water in a second stage to form a higher molecular weight polyamide.
Shyu et al. U.S. Pat. No. 4,739,035 discloses a two-step process for manufacturing polyamides from .alpha.,.omega.-dinitriles and .alpha.,.omega.-diamines in the presence of water comprising reacting the dinitrile and water in the presence of a catalyst under autogenous pressure and at a temperature sufficient to cause substantial hydrolysis of the dinitrile and subsequently adding the diamine to the resulting hydrolysate. The resulting reaction mixture is heated at a temperature sufficient to cause polymerization. Up to about 10 weight percent of the diamine may be present during the hydrolysis step. Addition of the major portion of the diamine after substantial hydrolysis of the dinitrile reduces the formation of triamines, for example bis(hexamethylene)triamine (BHMT) when the diamine is hexamethylene diamine. The polyamides formed had triamine levels in the range of 200 to 1,300 ppm compared to 1,420 to 1,610 ppm obtained using processes in which the diamine was added gradually over the course of the reaction. The presence of triamine causes branching and cross-linking of the polyamide which results in gelation and significant polymer quality deterioration.
Liehr U.S. Pat. No. 5,627,257 discloses a two-step process which claims to further reduce triamine formation. In the first step, the dinitrile is hydrolyzed under autogenous pressure until at least 85% of the hydrolysis product is dicarboxylic acid prior to addition of the diamine. The hydrolysis is conducted in a weakly acidic medium in the presence of a catalyst system of at least one oxygen-containing acid of phosphorus and/or at least one water-soluble calcium, zinc, manganese or cadmium salt. The pH of the hydrolysis reaction medium is adjusted by adding a sufficient amount of at least one saturated aliphatic or aromatic dicarboxylic acid. The dicarboxylic acid solution from the first step is reacted in a second step with at least an equimolar amount of diamine, expelling the resulting ammonia and water at an elevated pressure and temperature, and then polycondensing the mixture to yield a spinnable polyamide while gradually releasing the pressure. Triamine levels of between 500 and 600 ppm were achieved. A shortcoming of this process is that substantial amounts of co-catalyst are utilized. For example, the level of the dicarboxylic acid used ranged from about 1 to 13 weight percent based on the adiponitrile level.